The present invention relates generally to the field of satellite communications, and is specifically designed to increase the channel capacity of such a system utilizing two carriers per transponder by reducing the intelligible crosstalk between the two FDM/FM carriers.
In general, this crosstalk impairment is caused by amplitude modulation being imparted to a carrier as it passes through the bandpass filter preceding the non-linear amplifier. Due to the AM/PM characteristics of the non-linear amplifier, the amplitude fluctuation of this carrier will thus produce phase modulation in the other carrier. At the FM demodulator output of any particular carrier, a portion of this crosstalk is intelligible.
In existing FDM/FM satellite communications systems, two carriers per transponder are utilized because there are no commercially available modems having sufficient bandwidth to accommodate a single wideband carrier. When operating with two carriers per transponder, the transmitting power of the satellite transponder must be reduced (i.e., the satellite transponder traveling wave tube amplifier (TWTA) has an input backoff of approximately 12 dB) in order to prevent undesirable intelligible crosstalk between the two FM carriers.
In order to eliminate this crosstalk, prior art systems have used compensating filters between the FM modem output and the input to the non-linear high power amplifier for each carrier at the transmitting earth station. These filters have been used in order to linearize the response between the output of the earth station FM modulator and the input to the HPA. However, the transfer functions of such filters or equalizers do not account for the non-linearities inherent in the operation of both the HPA and the TWTA. Thus, such equalizers do not eliminate the intelligible crosstalk to the point where the nominal transponder TWTA operating point can be reduced less than minus 12 dB.